The present invention is especially applicable to the construction of a resolver; however, the invention, or at least certain aspects thereof, have application to other rotary signal generating devices. Consequently, the disclosure of the invention is not intended to be specifically limited only to a resolver.
In controlling various rotary devices with substantial positional precision, it is important to be able to determine the exact angular position of a rotary element, such as a shaft, at any given instant. This requirement becomes immensely more important as accuracy and automation demands increase. For this reason, substantial effort has been devoted for many years to a rotary device which will accurately determine and record the exact position of a rotary element within small tolerances. These efforts, and the demands to which they are directed, have resulted in extremely expensive signal generating devices for detecting and signaling the position of a rotary shaft. Such devices must be repeatable, and accurate. Errors of only ten to twenty minutes are unacceptable in certain applications. Such practical demands are somewhat inconsistent with the requirement that the device be fixed to a rotary element which may be eccentric and include a stationary element. Consequently, even mechanical misalignment and vibration and inertia can result in errors between the actual angle of an element being monitored and the reading of the resolver. This relationship of the constructed or measured angle as a function of the actual angle is referred to herein as the " error curve" of the resolver. The ability to reduce error or create an acceptable "error curve" and assure accuracy and repeatability from one resolver to the next while maintaining a competitive cost is an important commercial objective of any resolver design or improvement. Reduction of only a few minutes in the error curve of a resolver drastically enhances the overall acceptability of the resolver. Such reduction in error to specified limits is an objective which must be obtained while avoiding substantial increases in manufacturing cost of a resolver. To accomplish the level of error required in the field, relative accurate resolvers and synchros have been developed. However, they are expensive.
A resolver is a rotary transformer having a primary winding across which a fixed frequency ac exciting voltage is applied. Two secondary windings are orthogonally positioned to create a trigonometric sine and cosine induced voltage level reflective of the angle between the rotary element ("rotor") of the resolver and the stationary element ("stator") of the resolver. Rotation of the primary winding with respect to the orthogonal secondary windings, known as the sensing windings, creates voltages in the two output or sensing windings so that the relative magnitude of these two voltages can be detected and translated into an angular position between the rotor and stator. As an example, the voltage on one output winding, assuming a transformer ratio of 0.5 and a voltage of 4 volts on the exciting winding, is twice the cosine of the displacement angle. In this same example, voltage across the other winding is twice the sine of the same angle. In a two pole resolver, the sine and cosine voltage curves complete a cycle in 360.degree.. Of course, a four pole resolver would have sine and cosine voltage curves which create a total cycle in 180.degree.. This resolver would have two voltage cycles per revolution of the rotor with respect to the stator. The primary advantage of the resolver is the infinite resolution of the output voltages for any angular position of the rotor with respect to the stator. This angle sensing concept has been employed for a number of years and efforts have been exerted recently to increase the accuracy by reducing the error without a drastic increase in cost.
The present invention is particularly applicable to a resolver; however, certain aspects of the invention could be used in a synchro which is essentially as a resolver, except the sensing coil is a three phase winding spaced around the stator at 120.degree. increments. The resolver employs two groups of windings spaced from each other 90.degree. to produce the sine cosine output voltages previously described. The synchro uses three groups of windings.
For many years, both a resolver and synchro were excited by using slip rings for directing the fixed frequency ac voltage to the exciter winding on the rotor of the device. Use of slip rings created substantial mechanical difficulties and poor reliability. To overcome this disadvantage, some resolvers used a second transformer action wherein the A.C. exciter voltage for the rotor of the device was induced into a secondary winding on the rotor. A primary winding encircling this secondary winding carried an ac fixed frequency voltage that was induced into the secondary winding on the rotor. In this manner, the exciting voltage is applied to the rotor where it is employed for the purpose of exciting the orthogonal or three phase sensing windings carried on the surrounding stator.
The remainder of this discussion will relate to resolvers only; however, the discussion is equally applicable in many respects to synchros. In the past, resolvers often included internal and external stacks of laminations forming two facing pole pieces. One pole piece was on the stator and the other was on the rotor. The exciter winding was wound onto the internal rotor stack or pole piece and the orthogonal sensing windings were wound on the external stator stack or pole piece. Thus, the resolver was quite expensive and required complex manufacturing procedures and winding techniques for both the stator and rotor. The windings in such resolvers had variable pitches with a different number of turns in the various individual coils, which coils were spaced around the stator. The number of turns of the coils were chosen by a Fourier analysis based upon the variable pitch needed in winding the coils to eliminate higher harmonics in the output voltages and reduce the error curve. The "error curve" is, by definition, the difference between the actual rotor angle and the angle developed by the ratio of the sine and cosine output voltages. This "error curve" can be created by detecting the rotor angle with a precision encoder and comparing this actual angle with the created or imaginary angle from the sine/cosine processor. The sine/cosine processing and the Fourier analysis is performed by use of an appropriate computer. The measured error is processed by a Fourier processor to produce an output "error" for each of the harmonics, as well as for the fundamental. Only the low order harmonics are present since the windings essentially cancel all harmonics above the fourth harmonic.
Resolvers including a rotary exciting winding and a two phase output sensing winding network were used for many years. A substantial improvement of this concept was developed wherein a variable reluctance was employed for a resolver. The orthogonal sensing coils were wound around circumferentially spaced slots in an external stator. An internal rotor formed from a high permeability magnetic material and having two or more air gaps of different reluctances spaced circumferentially around the rotor was rotated within the stator to create variable reluctances from the many poles spaced around the stator. This structure produced an extremely accurate multi-pole brushless, transformless resolver. The exciting coils were also wound with respect to the pole pieces on the stator. Consequently, the fixed frequency A.C. exciting voltage and the two orthogonally displaced sensing voltages were wound onto the outer stator. A version of this concept is illustrated in Ringland U.S. Pat. No. 3,641,467 and Nagarkatti U.S. Pat. No. 4,631,510. By connecting the orthogonally positioned output coils in opposition, the fundamental, i.e. first harmonic, was essentially eliminated. Thus, the harmonic of concern was created by the combination of slots determining the spacing of pole pieces around the internal portion of the stator. These resolvers did not gain wide acceptance although they were inexpensive to manufacture and exhibited substantial potential. It was found that such variable flux resolvers have large electrical phase shifts between the excitation
voltage and the output voltages. There were extremely low harmonic output voltages for detecting the angular position. Consequently, such brushless, transformless resolvers functioning by variable reluctance are available, but are used only in limited applications. In recent years, as illustrated in the two above mentioned patents, improvements have been made to produce better output signals; however, they have still not been widely adopted since these devices had to be manufactured with precision, employing accurate bearings to control eccentricity and squareness. Further, such resolvers were difficult to mount on a pendulous shaft. Further, the excitation and output windings were connected in opposition. This reduced the effectiveness of the windings.
All of these disadvantages have been overcome by the new type of transformless brushless resolver disclosed in Luneau U.S. Pat. No. 4,659,953 and Luneau Re U.S. Pat. No. 32,857. In this new type of resolver or synchro, the stator did not include a single stack. There are two separate and distinct axially spaced stacks on the stator. These stacks produce inwardly facing pole pieces that are magnetically connected to produce a return magnetic path facing axially outward of the pole pieces. The exciting coil is wound concentric with the surface of the pole surfaces and between these surfaces. The rotor is a high permeability magnetic member that creates a low reluctance magnetic path that extends from one stack on the stator to the other spaced stack on the stator. The exciting coil produces a flux that emanates radially inward from one stack to the other. The rotor concentrates the flux appearing at one position on the first stack and connects this one position in a low reluctance path to a radially different position on the second stack of the stator. Thus, the rotor completes the magnetic path between the two axially spaced stacks or pole surfaces. As the rotor rotates with the shaft being monitored, the flux path created by the exciting winding rotates. In this manner, misalignment of the rotor causes no meaningful error in the detected voltages induced into the two orthogonal sensing windings mounted on the stator. As the rotor is turned, flux is rotated and changes the coupling to the sine and cosine windings in the stator, as in a conventional resolver configuration. Even though this new variable flux, transformless, brushless resolver has been a substantial improvement in the technology and is widely used for precision measurement of the position of a rotary element, such as a shaft, the unit is somewhat expensive to manufacture. In addition, substantial effort is required during the manufacturing to assure a satisfactory error curve. As tolerances for many applications are being reduced, even more pressure is applied to develop a resolver having the extremely advantageous concepts of the Luneau unit, but with reduced cost and reduced variations in the error curve even when the device is mass produced with a minimum of manual operations.